Macromolecular crowding dramatically affects cellular processes such
as protein folding and assembly, regulation of metabolic pathways,
and condensation of DNA. Despite increased attention, we still
lack a definition for how crowded a heterogeneous environment is
at the molecular scale and how this manifests in basic physical phenomena
like diffusion. Here, we show by means of fluorescence correlation
spectroscopy and computer simulations that crowding manifests itself
through the emergence of anomalous subdiffusion of cytoplasmic macromolecules.
In other words, the mean square displacement of a protein will grow
less than linear in time and the degree of this anomality depends
on the size and conformation of the traced particle and on the total
protein concentration of the solution. We therefore propose that
the anomality of the diffusion can be used as a quantifiable measure
for the crowdedness of the cytoplasm at the molecular scale.